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A leading Northland primary school principal says common sense would tell you if a child can't see they will not learn as well as they could.

So when free vision screening in Whangarei found a third of 290 children had an undiagnosed eye condition, Hora Hora Primary School principal Pat Newman said it was evidence there needed to be regular testing in schools.

"It's very worrisome because one of the problems we've got is low achievement in Northland and if a third of our kids are walking around needing glasses that perhaps might be an indication why," he said.

But Pat Tuohy, Ministry of Health chief adviser of Child and Youth Health, said the current testing - which includes at-birth screening, B4 School Check vision screening for amblyopia (lazy eye), and Year 7 (10 to12 year olds) vision checks at school which include distance visual acuity screening - was sufficient.

"Yes they are sufficient for finding children with amblyopia, however it's important they are regularly reviewed and updated as new evidence is made available.

"There's no evidence that more frequent screening leads to better health outcomes,"

The Essilor Vision Foundation, a New Zealand-registered charity, was in Whangarei for the first time last Friday vision screening 290 kids, aged about 8 to 11 years old, from low decile schools.

Of those 290 kids, 34 per cent had an undiagnosed eye condition ranging from hyperopia (long sightedness) and astigmatism ('rugby-ball' shaped eyes) to myopia (short sightedness) and binocular imbalances (where the eye muscles are under strain with near work).

Dr Tuohy said those statistics were not surprising as B4 School Checks do not specifically screen for those conditions as they do not lead to amblyopia.

"Many of them will have no impact of a child's health and development," he said.

Mr Newman said common sense indicates if a child can not see, they will not learn.

"They'll have headaches, they won't be able to see. Where the hell is common sense? If a kid can't see, then you don't learn," he said.

Mr Newman said teachers thought if children had gone through the early screenings they would be fine, but the result the free eye testing showed that was not true.

"I think it clearly points to the current testing regime in primary schools has failed and there needs to be one urgently replaced so that every child is checked regularly at least twice during their primary school time, in schools, and it should be a matter of right."

He said the idea that these eye conditions may not have been identified without The Essilor Vision Foundation's testing was scary.

"It's more than frustrating because again we are showing that our kids are missing out."

Anita Pistorius, from Lowes & Partners Optometrists, said one of the reasons these eye conditions may have gone unidentified because it is difficult for kids to know that their eyesight isn't normal.

Kumuda Setty, from Essilor Vision Foundation, said once the children are vision screened an assessment is made to decide whether the child needs to be referred for a full eye exam.

Schools make arrangements to take the kids to the optometrist - if they need glasses and are eligible for the Ministry of Health's children spectacle subsidy Enable, one pair will be billed under that and the other provided free-of-charge to be kept in school.